There is a growing desire to use multimedia broadcasts/multicast services (MBMS) in wireless communication systems. For a particular MBMS, a given cell in the network may have none, one or multiple users, wireless transmit/receive units (WTRUs), subscribed to that MBMS. As users migrate between cells, a cell that may originally have one or no users subscribed to the service may at a later point have multiple subscribed users. Conversely, a cell that may at one point have multiple subscribed users may at another time have only one or no users.
This migration of the users can create inefficient use of radio resource. If only one or a few users are subscribed to the MBMS in the cell, it may be desirable to support the service using dedicated channels to the users. The dedicated channels can utilize power control and beam steering to reduce the amount of radio resources utilized to support the MBMS.
However, if many users are subscribed to the MBMS in the cell, the multiple dedicated channels in total may use considerable radio resources. In such a situation, a more optimal use of the radio resources may be to send the MBMS data over a common/shared channel to a set of users subscribed to that MBMS. Although the use of beamsteering and power control in such situations is limited, the reduction in the number of total channels may reduce the used radio resources. As the number of MBMS users in the cell changes, an original choice of using either a common/shared channel or a dedicated channel may not be optimal at a later time.
Another potential problem resulting from user migration is MBMS scheduling. As a MBMS user moves between cells, that user needs to reconstruct the MBMS service from information received from both cells. If both cells synchronize their MBMS transmissions, the MBMS user can seamlessly move between the cells. However, such a scenario is typically not practical and undesirable. At certain time intervals based on the cell's loading and available resources, a cell may have more available resources to support the MBMS transmission than at another time. As a result, at that time, it is desirable for that cell to transmit a large amount of the MBMS data. For another cell at that same time interval, resources to support the same MBMS bandwidth may not be available. As a result, it may be desirable to schedule the MBMS transmissions differently between the cells, to better utilize such resources. As the MBMS user moves between cells, the new cell that the user has moved into may have transmissions either ahead or behind the other cell's transmissions. As a result, the MBMS user may miss MBMS data or needlessly receive redundant MBMS data.
Accordingly, it is desirable to have better resource utilization for MBMS.